Process for producing a copolymer of tetrafluoroethylene and propylene using radiation and a solvent

ABSTRACT

COPOLYMERS OF TETRAFLUOROETHYLENE AND PROPYLENE ARE PREPARED IN A NON-POLYMEPIZABLE MEDIUM OF A SOLVENT OR SWELLING AGENT, USING HIGH ENERGY IONIZING RADIATION.

United States Patent O 3,723,270 PROCESS FOR PRODUCING A COPOLYMER OF TETRAFLUOROETHYLENE AND PROPYLENE USING RADIATION AND A SOLVENT Yoneho Tabata, Matsudo Chiba, and Gen Kojima, Tokyo, Japan, assignors to Japan Atomic Energy Research Institute,Tokyo, Japan No Drawing. Filed Dec. 11, 1970, Ser. No. 97,386 Int. Cl. C08d 1/00; C08f N16 US. Cl. Z04159.22 4 Claims ABSTRACT OF THE DISCLOSURE Copolymers of tetrafluoroethylene and propylene are prepared in a non-polymerizable medium of a solvent or swelling agent, using high energy ionizing radiation.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a process for copolymerizing tetrafluoroethylene and propylene, and more particularly to a method for copolymerizing tetrafluoroethylene and propylene with high energy ionizing radiation in the presence of a solvent or swelling agent.

Description of prior art It has been known to produce copolymers of tetrafluoroethylene and propylene by suspension polymerization of tetrafluoroethylene and propylene in the presence of an organic peroxide catalyst, as disclosed in 'British Pat. 594,249, or by emulsion polymerization of tetrafluoroethylene and propylene in the presence of a water soluble catalyst, as disclosed in US. Pat. 3,467,635. These prior art processes, however, have the disadvantage of requiring high pressures, about 150 kg./cm. which necessitates the use of complex and heavy equipment. Moreover, the polymerization reaction rate of these prior art processes is too slow, and the purification of the copolymer is diflicult. It has also been known to produce copolymers of tetrafluoroethylene and propylene with high energy ionizing radiation, see Kogyo Kagaku Zasshi 68 (1 1926-9, to produce a pure product. However, that process also had the disadvantage of a slow polymerization reaction rate, e.g., as low as 0.1% per hour, and a low degree of polymerization. Furthermore, it is quite diflicult to remove the heat of reaction from that process so that it is not commercially feasible.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a novel process for obtaining copolymers of tetrafiuoroethylene and propylene at high polymerization reaction rates.

It is another object of this invention to provide a novel process for obtaining a relatively pure copolymer of tetrafluoroethylene and propylene at high polymerization reaction rates to form a product having a very narrow distribution of molecular weight.

It is another object of this invention to provide a process for producing a relatively pure copolymer of tetrafiuoroethylene and propylene having a very narrow distribution of molecular weight under relatively low pressure and low temperature conditions.

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These and other objects have now herein been attained by irradiating a mixture of tetrafluorethylene and propylene with high energy ionizing radiation in a medium containing a solvent or swelling agent for the resulting copolymer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The molar ratio of tetrafluoroethylene to propylene can be selected over a wide range. For instance, suitable results are attainable when the molar ratio of tetrafluoroethylene/ propylene is from 99/1 to 1/ 99, preferably 99/1 to 30/70, and especially /5 to 50/50. The degree of polymerization and the particular composition of the copolymer, of course, will depend upon the particular molar ratio of the tetrafluoroethylene used. When higher concentrations of tetrafluoroethylene are used, the molecular weight of the resulting copolymer has been found to be sufiiciently high and the copolymerization reaction rate has been found to be suitable. Moreover, the solvent resistance of the copolymer is high.

The reaction medium must be selected according to certain specific criteria:

(a) The medium should be capable of accelerating the polymerization reaction rate by initiating the reaction or by accelerating the propagation of the reaction. For example, diethylether or tetrahydrofuran in the reaction medium will result in a copolymerization reaction rate which is substantially the same as that obtainable with conventional processes. That is, the reaction rate will be the same as if no special medium were present at all. Accordingly, these solvents are not suitable for the present invention.

(b) The medium should not hinder the production of free radicals of tetrafluoroethylene or propylene, which is the mechanism for the radiation copolymerization reaction. For example, the use of benzene or toluene Will retard the formation of free radicals, thereby resulting in a lower copolymer yield.

(0) The medium should not be chain transferable with tetrafluoroethylene or propylene under the action of high energy ionizing radiation, since the molecular Weight of the resulting copolymer will be decreased thereby possibly resulting in a liquid copolymer. Chloroform, carbon tetrachloride and dichloroethane each possess chain transfer properties and accordingly should not be used in the reaction medium.

(d) The medium should be a liquid under the reaction conditions.

(e) The medium should dissolve at least one monomer, either the tetrafluoroethylene or the propylene, to yield a homogeneous copolymerization reaction system. For example t-butanol will dissolve only propylene, but fluorohydrocarbons and fluorochlorohydrocarbons will dissolve both tetrafluoroethylene and propylene.

(f) The medium should be either a swelling agent or a solvent for the resulting copolymer, but should be nonpolymerizable under the reaction conditions. Fluorohydrocarbons or fluorochlorohydrocarbons are good solvents and t butanol is an acceptable solvent, although weak.

(g) The medium should have a boiling point of between 10 C. to +100 C.

When the reaction medium is selected using the above criteria, a high molecular weight copolymer having a narrow distribution of molecular weight can be effectively produced at a high rate of copolymerization. Moreover, a homogeneous copolymerization reaction system is obtained which provides a higher degree of reproducibility.

Fluorohydrocarbons and fluorochlorohydrocarbons containing 1 to 4 carbon atoms, have been found to be especially suitable mediums for the present invention. Suitable mediums include the chlorofiuorohydrocarbons, such as monofiuorotrichloromethane, tn'fiuorotrichloroethane, tetrafluorodichloroethane; fiuorohydrocarbons, such as perfluorocyclobutane. n-Butanol and t-butanol can also be used as the medium, although the molecular weight distribution of the resulting copolymer will be a little broader compared with that obtained when copolymerization is effected in the presence of a chlorofluorohydrocarbon or a fluorohydrocarbon. n-Butanol and t-butanol can dissolve the monomers, but they will only slightly swell the resulting copolymer. Although tetrahydrofuran is a good solvent for the copolymer, it has an adverse effect in blocking the copolymerization reaction and hence is unsuitable. Likewise, benzene tends to deactivate the copolymerization reaction, and accordingly is likewise not suitable.

The molar ratio of the monomers to the medium depends upon the particular medium, reaction temperature, molar ratio of tetrafiuroethylene to propylene and type of apparatus used. Good results are obtainable when the molar ratio of monomer to medium is between 1/10 to 10/1 and preferably between 1/5 to 10/1. Most preferably, best results are obtainable when the molar ratio is between 1/2 to 6/1. The molar ratio of the monomers to the medium will affect the rate of copolymerization reaction, the molecular weight and the distribution of the molecular weight. In general, however, it will not affect the ratio of C F /C H in the resulting copolymer.

Any type of high energy ionizing radiation can be used to effect the copolymerization reaction. For instance, 'yrays, X-rays, a-rays, fi-rays, or electron rays may be used. The dose rate of the ionizing radiation may be between 10 -10 roentgens per hour and preferably 10 -10 roentgens per hour. In general, when a high dose rate is used, the copolymerization reaction rate will be high, but the molecular weight of the resulting copolymer will be low. The temperature of the copolymerization reaction is not critical and can be carried out within a range of from 40 C. to +150 C., and preferably 20 C. to +100 C.

The pressure of the reaction can be lower than those of conventional processes. Good results are obtainable when the pressure is less than 100 kg./cm. preferably 1 to 50 kg./cm. and most preferably between 5 to 30 kg./cm.

One of the advantages of the present invention is that a very narrow distribution of the molecular weight is obtained. To this end, the following conditions should be observed:

(a) Molar ratio of C F /C H The distribution of the molecular weight will be broadened if the quantity of tetrafluoroethylene is too high.

(b) Ratio of M /S (monomers/medium): The distribution of the molecular weight will be broadened if the ratio of M/ S is more than 50/50.

(c) Dose rate. The distribution of molecular weight will be narrowed if the dose rate of irradiation is between 10 to 10 roentgens per hour.

(d) Reaction period: The distribution of molecular weight will be broadened if the reaction period is too long.

In the process of this invention, tetrafluoroethylene and propylene are respectively charged to a reactor at a predetermined ratio from pressurized containers. The med ium is charged to the reactor system and the contents of the reactor are repeatedly solidified and de-aired. In commercial scale, the autoclave is de-aired by purging with nitrogen gas.

The mixture is then irradiated with high energy ionizing radiation at a predetermined dose rate for a suitable period of time. The resulting copolymer of tetrafiuoroethylene and propylene is obtained by discharging the unreacted monomer and the medium as a gas and by drying the medium using vacuum techniques.

If preferable, the resulting product can be purified by dissolving in a solvent such as tetrahydrofuran and precipitating from methanol.

The reaction is usually effected under relatively low pressures, such as 10 atmospheres, although the reaction mixture should be in a liquid phase.

The copolymerization of the present invention is very advantageous for commercial adaptation for a number of reasons. The copolymerization reaction rate is quite high and the inherent viscosity of the resulting copolymer is high. The reaction temperature is quite easily controlled since heat is rapidly removed through the medium. The reaction can also be effected over a wide temperature range of from 40" C. to 150 C., although it is usual to effect the reaction at about 0 C. Purification of the resulting product is not difficult, and the properties of the copolymer can be easily altered by selecting the molar ratio of tetrafiuoroethylene to propylene. The molecular weight distribution of the copolymer can be easily controlled by controlling the radiation dose rate.

Having generally described the invention, a further understanding can be attained by reference to certain specific Examples which are provided herein for purposes of illustration only and are not intended to be limiting in any manner.

EXAMPLE 1 21.4 g. of tetrafiuoroethylene, 5.1 g. of propylene, and 63.7 g. of trifluorotrichloroethane were charged into an autoclave made of stainless steel and having a volume of cc. The air was removed by repeated melting-solidifying-evacuating and the autoclave was cooled with ice water to a temperature of 0 C. 'y-rays from a source of cobalt 60 was used to irradiate the mixture and was applied at a dose rate of 3.0 10 roentgens/hour for 5 hours. Unreacted monomer was discharged with the trifluorotrichloroethane and 10.3 g. of a copolymer of tetrafluoroethylene and propylene was obtained. The polymerization reaction rate was 7.8%/hr., and the tetrafiuoroethylene content in the copolymer was 49 mole percent. The inherent viscosity of the copolymer in tetrahydrofuran solution (100 cc./g.) at 30 C. was 0.19.

Reference Example 1 The process of Example 1 was repeated except using 74.4 g. of tetrafluoorethylene, 15.6 g. of propylene. No medium was included during irradiation. The mixture was irradiated for 10 hours and 19.1 g. of tetrafiuoroethylenepropylene copolymer was obtained. The copolymer was found to contain 49 mole percent tetrafiuorethylene. The polymerization reaction rate, however, was only 1.9%/ hour, and the inherent viscosity in tetrahydrofuran (100 cc./g.) at 30 C. was 0.12.

EXAMPLES 2-8 Tetrafluoroethylene, propylene, and reaction medium were charged into a 100 cc. volume autoclave made of stainless steel. Air was released from the reactants by repeated melting-solidifying-deairing and the autoclave was maintained at a specific temperature for further reaction. 'y-rays from a cobalt 60 source was used to irradiate the mixture. Unreacted monomer was discharged with the medium to obtain the copolymer. The yield, content of tetrafiuoroethylene, polymerization reaction rate, and inherent viscosity of the copolymer in tetrahydrofuran, as well as the appearance of the copolymer are stated in Table 1.

TABLE 1 Monomer content Copolymer Radiation TFE polymeri- TFE Medium conzation P 1 Dose tent reaction Inher- Example TFE 1 P 1 (molar mount rate Temp. Time Yield (perrate (perent visnumber (g.) (g.) ratlo) Type (g.) (y/hr.) 0.) (hr.) (g.) cent) cent hr.) cosity Mechanical property 27. 8 5. 6 2/1 MFTCM 51. 5 3. X10 5 12. 5 40 7. 5 0. 18 21. 2 4. 6 2/1 PFCB 64.0 3. 0X10 5 4. 8 50 3. 7 0.22 Rubber-like elasticity. 21. 4 5 1 2/1 TFTCE- 63. 7 3. 0X10 5 2. 6 48 2. 0 0. 09 21. 4 5. 1 2/1 TFTCE 63. 7 3. 0x10 1 12. 8 51 50. 0 0. 23 Rubber-like elasticity. 32.3 1. 4 9/1 TFTCE 66.3 3. 0X10 0.75 5.1 54 20.0 0.5 Do. 21.4 5.1 2/1 TFTCE. 63.7 4.4X10 10 1.3 49 0.5 0.3 Do. 21.4 5. 1 2/0 t Blutyll 1 48. 0 4. 4X10 10 21. 2 50 8. 1 0. 3 Do.

a co 0 1 TFE te trafluoroethylene. P propylene NOTE: MFTCM =monofluorotrichloromethane; PF CB =pcrtluorocyclobutane; TFT CE triflnorotrichloroethane:

In order to show the advantages of this invention as compared to the same reaction occurring in the absence of the medium, experimental results both with and without the addition of the medium are shown in Tables 2 and 3.

EXPERIMENT 1 Tetrafluoroethylene and propylene were copolymerized in a 25 cc. ampoule. The conditions of reaction temperature, molar ratio of C F /C H molar ratio of monomer to medium, dose rate of 'yrays and time of irradiation are stated in Table 2. The results of yield in weight percent, number average molecular weight (hereinafter referred to as Mn), weight average molecular weight (hereinafter referred to as MW), molecular weight distribution (hereinafter referred to as MW/Mn), and inherent viscosity of When t-butanol was used, Mw/Mn was 2.26, which shows a rather broad molecular weight distribution, although the molecular weight itself was quite high. When no medium was used, MW/fil'l was quite high, which shows a broad distribution of molecular weight. The resulting copolymer does not dissolve in the monomer, and copolymerization is effected in a two-phase system. On the other hand, when the resulting copolymer is dissolved in the medium, copolymerization occurs in a single-phase system.

EXPERIMENT 2 Tetra fluoroethylene and propylene were copolymerized under 'y-rays in a five liter autoclave.

The conditions of reaction and the results are shown in Table 3.

TABLE 3 Irradiation Yield Tempera- C2F /CaHe Monomer} Dose rate time (weight Mn MW Medium ture 0.) molar ratio medium (r/hr.) (hrs.) percent) (X 10,000) (X 10,000) Mw/Mn TFTCE 1 0 75/25 4 5X10 25. 8 3. 4 7. 3 2. 17 0. 42 'IFTCE 1 0 75/25 4 5X10 20 34. 4 3. 4 7. 4 2. 21 0. 47 TFTCE L 0 75/25 4 5X10 20 30. 6 3. 5 8. 0 2. 28 0. 37 TFTCE l 0 75/25 4 5X10 8 11.2 2. 7 4. 7 1. 76 0. 33 TFTCE 0 75/25 4 5X10 20 36. 8 3.0 6.0 2. 04 0. 38 t-Bl1t81101 0 75/ 4 5X10 17 17. 7 2. 9 7. 2 2. 48 0. 40 None, medium 0 90/10 4. 5X10 18 7. 9 0. 73 3. 5 4. 80 0. 69 D0 28 6/34 4X10 17 34. 1 2. 9 9. 0 3. 16 0. 44

the copolymer in tetrahydrofuran ([1 are also shown in Table 2. The molecular weights were measured by gel permeation chromatography (apparatus manufactured by Waters Associates (30.). The molecular weight of the copolymer is preferably greater than 20,000, which corresponds approximately to more than 0.2 of [1 inherent viscosity.

EXPERIMENT 3 Tetrafiuoroethylene and propylene were copolymerized in a stainless steel autoclave or an ampoule, using high energy 'y-rays. In order to show the effect of molar ratio TABLE 2 Irradiation Yield Tempera- CZF4IC3H6 Monomer] Dose rate time (weight Mn MW Medium ture C.) molar ratio medium (rlhr.) (h percent) (X 10,000) (X 10,000) Mw/Mn 1 66/34 1 5. 0X10 4. 0 13. 4 2. 5 4. 3 1. 73 0. 26 1 66/34 1 5. 0X10 9. 0 27. 6 2. 4 4. 2 1. 73 0.25 1 66/34 1 5. 0X10 13. 5 41. 7 2. 3 4. 0 1. 73 0. 26 1 90/10 1 5 0X10 2. 5 11. 9 3. 2 5. 9 1. 83 0. 31 1 66/ 34 1 2. 2X10 6. 0 7. 3 2. 7 4. 8 1. 74 O. 35 75 66/34 1 5. 0X10 5. 0 42. 7 2. 4 4. 1 1. 72 0. 25 1 66/34 4 5. 0X10 --7. 5 17. 0 3. 0 5. 5 1. 83 0. 27 1 66/34 1 3. 0X10 5. 0 40. 7 2. 6 5. 8 2. 26 0. 35 23 10. 0 14. 5 2. 2 6. 5 2. 92 0. 30 -23 5. 5 10. 5 2. 8 8. 9 3. l7 0. 54 23 6. O 5. 3 3. 6 12. 9 3. 0.71

1 TFTC E =trlfluorotrichloroethane.

Mw/Mn shows the distribution of molecular weight. A random distribution would be indicated as Mw/Mn=2.0. As shown in Table 2, Whenever trifluorotrichloroethane was used, Mw/Mn was less than 2.0, which shows a very narrow molecular weight distribution.

and ratio of monomers to medium, the following experiments were conducted. In each experiment, trifluorotrichloroethane was used as the reaction medium and the reaction temperature was maintained at 0 C. The dose rate of 'y-rays was 5x10 roentgens/hour.

TABLE 4 Irradiation Yield (wt. MW]

Number CzF4/C3H0 M/S time percent) MnX10- MnX10- Mn CzF4/C3H0 00/34 4 8 17 2. 2 44 2.00 0. 24 50/50 75 25 4 20 20 a. 5 8.0 2. 2s 0. 37 51 49 90 4 5 11 2.5 5.8 2.3 0.31 54/46 a 10 4 10 2a 3.2 8.3 2.0 0.44 55 45 00 34 1/4 13 30 1.8 3.2 1.8 0.20 51/40 60/34 1/1 a 10 2.5 4.3 1.1 0.20 50 50 Mn=4.6 (X10000).

I Mu=5.5 (x10,000).

In Experiments 1, 2, 5, and 6, a 20 cc. ampoule was will not prevent the formation of free radicals, which is used as the reactor. In Experiments 3 and 4, a 5 liter autocharacterized by a low chain transfer action and which clave was used as the reactor. is capable of dissolving at least one of the monomers In Experiment 4, propylene monomer was added to 15 and swelling or dissolving the resulting copolymer, wherethe tetrafluoroethylene over a period of 5 hours to obtain in the reaction is effected at a temperature of -40 the predetermined ratio of C F /C H =90/ 10. The reacto '+l50 C., a pressure of 1 to 50 kg./cm. and wherein tion was continued for 5 additional hours. The value of the molar ratio of tetrafluoroethylene/propylene is from fin measured by gel permeation chromatography was 99/1 to 1/99. found to be less than the actual value of the weight avere pr of Claim wherein e molar ratio of age molecular weight. said tetrafiuoroethylene and propylene monomers to said When no reaction medium was used, fiw/Mn was non-polymefilable medium is 1/10 to 10/1- found to be quite high, which shows a broad distribution 3. The Process of Claim 1, wherein the molar ratio of of molecular weight. On the other hand, when trifluorosaid monomers of tetrafluoroethylene and propylene to trichloroethane was added to the medium, Hw/Mn was said non'polymel'izable {medium is to found to be about 2.0, even though the molecular weight The Process of Glenn Wherem the molar who of was quite high. These results show that a very narrow tetfafluol'oethylene t0 P py is 95/5 to 50/50- distribution of molecular weight is obtained.

When t-butanol was used, Hw/fin was found to be References C'ted 2.48, which is quite low in comparison to the broad UNITED STATES PATENTS distribution of molecular weight which occurs when no 3,437,648 4/1969 Dietrich R medluIP 18 used- 3,467,635 9/1969 Brasen et al. 260-80.76

Havmg now fully descnbed the 1nvent1on, 1t w1ll be 3,058,899 10/1962 Yanko et aL 204 159 22 apparent to one of ordlnary Sklll 1n the art that many 3 342 777 9/1967 Howard Jr 22 changes and modifications can be made without departn ing from the spirit or scope of the invention. OTHER REFERENCES Accordingly, What is claimed as new and desired to Radiation Induced Copolymerization of TFE with Probe secured y Letters Patent 0f the United States i pylene at Low Temperature, Tabata et al.: J. of Polymer In a process for producing copolymers of tetra- 40 Science, Part A, vol. 2, pp. 2235-2243 (1964). fiuoroethylene and propylene whereby tetrafluoroethylene is copolymerized with propylene and high energy ioniz- MURRAY TILLMAN, Primary Examiner ing radiation, the improvement comprising effecting said R B TURER Assistant Examiner reaction in a medium selected from the group consisting of a chlorofluorohydrocarbon and fiuorohydrocarbon, us CL which is non-polymerizable, which is capable of accelerating the rate of the copolymerization reaction, which 

